hashkey/digest for a complex object

Arnaud Delobelle arnodel at gmail.com
Thu Oct 7 22:00:16 CEST 2010

kj <no.email at please.post> writes:

> The short version of this question is: where can I find the algorithm
> used by the tuple class's __hash__ method?
> Now, for the long version of this question, I'm working with some
> complext Python objects that I want to be able to compare for
> equality easily.
> These objects are non-mutable once they are created, so I would
> like to use a two-step comparison for equality, based on the
> assumption that I can compute (either at creation time, or as needed
> and memoized) a hashkey/digest for each object.  The test for
> equality of two of these objects would first compare their hashkeys.
> If they are different, the two objects are declared different; if
> they match, then a more stringent test for equality is performed.
> So the problem is to come up with a reasonable hashkey for each of
> these objects.  The objects have two significant attributes, and
> two of these objects should be regarded as equal if these attributes
> are "the same" in both.  The first attribute is a simple dictionary
> whose keys are integers and values are strings.  The second attribute
> is more complicated.  It is a tree structure, represented as a
> dictionary of dictionaries of dictionaries... until we get to the
> leaf elements, which are frozensets of strings.  The keys at every
> level of this structure are strings.  E.g. a simple example of such
> an attribute would look like:
> {'A': {'a': set(['1', '2', '3']),
>        'b': set(['4', '5'])},
>  'B': set(['6', '7', '8'])}
> I'm looking for a good algorithm for computing a hash key for
> something like this?  (Basically I'm looking for a good way to
> combine hashkeys.)

You could do something like this:

deep_methods = {       
    list: lambda f, l: tuple(map(f, l)),
    dict: lambda f, d: frozenset((k, f(v)) for k, v in d.items()),
    set: lambda f, s: frozenset(map(f, s)),
    # Add more if needed

def apply_method(f, obj):
        method = deep_methods[type(obj)]
    except KeyError:
        return obj
    return method(f, obj)

def deepfreeze(obj):
    """Return a 'hashable version' of an object
    return apply_method(deepfreeze, obj)

def deephash(obj):
    """Return hash(deepfreeze(obj)) without deepfreezing"""
    return hash(apply_method(deephash, obj))

# Example of deepfreezable object:
obj = [1, "foo", {(2, 4): {7, 5, 4}, "bar": "baz"}]

>>> deepfreeze(obj)
(1, 'foo', frozenset({('bar', 'baz'), ((2, 4), frozenset({4, 5, 7}))}))
>>> deephash(obj)
>>> hash(deepfreeze(obj))


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